FIG. 32 is a sectional view of a conventional thermal fuse. The conventional thermal fuse shown in FIG. 32 includes fusible alloy 2 including tin and a pair of lead conductors 3 connected to respective ends of the alloy 2. The connectors may be connected by welding or supersonic welding, or by melting the fusible alloy 2 with an electrical power applied to the lead conductors 3 and the fusible alloy 2. The fusible alloy 2 is coated with flux 4, and accommodated in insulating case 1 having an opening. The opening of the insulating case 1 is sealed with sealer 5 made of hard resin.
In the conventional thermal fuse, the flux 4 melts according to an increase of an ambient temperature, thus removing an oxide from a surface of the fusible alloy 2. Then, when the ambient temperature further increases and exceeds the melting point of the fusible alloy 2, the fusible alloy 2 melts to break electricity. For reliably achieving the fusing, the flux 4 is applied to a wide portion of the entire surface of the fusible alloy 2. When the thermal fuse melts, arc is generated between respective tips of melting fusible alloy 2. In particular, when a high voltage or a large current is applied, energy of the arc is increased. The arc energy vaporizes or decomposes the flux 4 provided on the fusible alloy 2. This rapidly increases the number of molecules in the insulating case 1 sealed with the sealer 5, thus increasing a pressure in a space in the thermal fuse. If the energy of the arc is large, the energy may cause the sealing to deteriorate or otherwise damage the insulating case 1 of the thermal fuse. Therefore, the conventional thermal fuse cannot be used as a thermal fuse for breaking a high voltage or a large current.